A new study provides further evidence that a simple scratch-and-sniff test could predict Parkinson's disease even earlier than previously thought.

According to Michigan State University researcher Honglei Chen, lead author and professor of epidemiology, the test could identify certain people who are at an increased risk of developing the disease up to 10 years before they are actually diagnosed. Previous research has shown an association between sense of smell and disease progression of up to four to five years.

People with poor sense of smell were nearly five times more likely to develop the disease than people with a good sense of smell. Of the 764 people with a poor sense of smell, 26 people developed the disease, compared to just seven of the 835 people whose sense of smell was good and nine of the 863 people whose sense of smell was categorized as medium.

Researchers also discovered that the results stayed the same after adjusting for other factors that could affect risk including smoking, coffee intake and history of head injury.

"It's important to note that not everyone with low scores on the smell test will develop Parkinson's disease," Chen said. "More research is needed before the smell test can be used as a screening tool for Parkinson's, but we are definitely on to something and our goal now is to better characterize populations that are at higher risk for the disease and to identify other factors involved."